1. Field of the Invention
The invention relates to a method of sensing the presence or absence of an article at the end of a vacuum pick-up means which is connected to a vacuum source for the picking up, transport and placement of an article, the article being picked up and retained by the pick-up means under the influence of the vacuum and being released when the vacuum is interrupted or removed, the difference in the vacuum occurring in the presence and the absence of the article being used to generate a test signal.
2. Description of the Prior Art
For the picking up, transport, transfer and placement of articles by means of vacuum pick-up means it is desirable, if not necessary, to know:
a. that the article has indeed been attracted and picked up by the pick-up means;
b. that the article remains attached to the pick-up means over the entire transport path until the instant at which the article is placed;
c. that the article has indeed been placed and left behind in the appropriate location.
Detection and sensing of the presence or absence of an article at the end of the pick-up means can be performed in various ways, for example, by direct detection by means of sensors, by detection of a relative displacement of the pick-up means by mechanical means or by optical means, or by indirect detection by means of a pressure detector or a flowmeter.
A method of the kind set forth is known from U.S. Pat. No. 3,453,714, which discloses an indirect detection method utilizing a diaphragm which reacts to a difference in vacuum. A further indirect method which utilizes a differential pressure switch is known from U.S. Pat. No. 4,116,348. Detection on an optical basis by means of a photocell (again an indirect method) is known from U.S. Pat. No. 3,731,867. Finally, European patent application No. 36.826 discloses a detection method which utilizes a pressure detector as well as an optical detection method. The known detection methods either use a displaceable member and/or react to a differential pressure.